CFD Simulation of Laminar Liquid Film Condensation On the Entrance Region of Vertical Plate
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A computational fluid dynamics model based on volume of fluid method is developed to investigate the laminar film condensation accompanied by mixed convection of humid air at the entrance region of a vertical flat plate. The effects of vapor in different concentration ranges are taken into consideration in this research. The vapor concentration range selected is less than 10% and more than 95%, respectively. It is demonstrated that the condensation rate (mcond) and heat transfer coefficient (hcond) depend greatly on inlet vapor concentration (wv0), and they vary significantly at a short distance near the inlet, which indicate the importance of the entrance region.
At low vapor concentration range, mcond and hcond increase with the increasing of wv0. Latent heat transfer (Ql) is dominant in the process of the interface heat transfer process, and is about 100 times larger than sensible heat transfer (Qs). Therefore, the latent heat transfer occurring in the gas-liquid interface is the dominant form in the heat transfer process. The heat generated by condensation enhances the liquid film temperature, which limits the sensible heat transfer. Moreover, both of them decrease rapidly along the vertical direction, and reduce half or more in a 50 mm distance.
While, at high vapor concentration range, mcond with higher wv0 is larger at the upstream half of the plate, and is smaller than that with lower wv0 near the outlet. This is the reason that more heat generated by condensation with higher vapor concentration greatly increases the interface temperature in the lower part. It enhances the saturated vapor concentration at interface, and reduces the concentration gradient between the interface and gas bulk. In addition, the extreme value of hcond can be observed in specific position in the high vapor concentration range. Due to the heat generated by condensation, the interface temperature is higher than that of the liquid film and gas phase nearby, and the temperature of the liquid film can also be higher than that of the gas phase. The higher temperature of liquid film leads to a negative value of Qs, which means the heat begins to transfer from liquid film to gas buck.